468 



Special Vertebrate Organogenesis 



(O'Connor, '40), but may also join other 

 regions of the gut (O'Connor, '39; Holtfreter, 

 '44). 



THE DEVELOPMENT OF THE 

 MESONEPHROS 



Up to the middle neurula stage at least, 

 while the mesonephric material is still in 

 process of involution, determination within 



act as an inductor (for a discussion see Griin- 

 wald, '37). Experiments in which develop- 

 ment of the duct was prevented in the meso- 

 nephric region provide a direct test of this 

 hypothesis (Fig. 176). The results, however, 

 vary greatly and are inconclusive. In most 

 cases only rudiments of tubules, or mere cel- 

 lular condensations, appear in the absence of 

 the diict (Fig. 1755); and in one species no 

 visible blastema develops. But if the duct is 



Fig. 1 76. Table showing the effects of absence of the nephric duct on the differentiation of the tubules of 

 the mesonephros. In most cases tubules fail to differentiate, although local cellular condensations may 

 appear; in some species, however, variable development of tubules may occur, subject to later degeneration. 



the area is labile to the extent that foreign 

 material is still readily assimilated (Spof- 

 ford, '48). Subsequently two degrees of auton- 

 omy can be demonstrated. In later neurula 

 stages mesonephric tissue placed in ectopic 

 situations (coelom, eye chamber) is capable 

 of producing unspecific tubular structures, 

 but is still plastic to a degree — at the site 

 of the pronephros it precociously forms 

 tubules of pronephric type (Machemer, '29). 

 In this phase, rate of development and final 

 character of morphogenesis are still modified 

 by external forces. After the tail-bud stage 

 determination is fixed. The same material 

 produces typical mesonephros in any loca- 

 tion (Humphrey, '28a); and nephrogenic 

 potency is restricted to the prospective meso- 

 nephric material (e.g., van Geertruyden, 

 '46: Cambar, '48; cf. also Gruenwald, '42). 

 The mesonephric tubules originate from 

 nephrotomes or as condensations in the un- 

 segmented nephrogenic cord (p. 462) long 

 after the nephric duct has developed — -a fact 

 which long ago suggested that the duct might 



removed after brief contact with the nephro- 

 genic tissue, delayed but fairly complete dif- 

 ferentiation occurs (O'Connor, '39). Again, if 

 the duct is displaced, tubules develop only 

 where nephrogenic tissue remains in close 

 proximity to the duct (Cambar, '48). In such 

 cases the role of the duct seems clear. In 

 other cases, however, the tubules behave iv 

 an irregular fashion, showing considerable 

 autonomy. All stages of differentiation may 

 be realized in the absence of the duct, even 

 in the same species or individual (e.g., Shi- 

 masaki, '30b; Humphrey, '28a,b; Burns, '38; 

 Gruenwald, '42; Nieuwkoop, '48). The im- 

 portance of the duct may well vary in differ- 

 ent species, or other inductors may be in- 

 volved (see Gruenwald, '42, '43; van Geer- 

 truyden, '46). Account must be taken also of 

 the fact that all tubule primordia (even in 

 the same region are not of the same morpho- 

 logical order (e.g.. Hall, '04; Gray, '32) and 

 mav possess different capacities for self- 

 differentiation. These questions require 

 further investigation. 



